Cloning and immunolocalization of a rat pancreatic Na+ bicarbonate cotransporter

In the rat, pancreatic HCO3- secretion is believed to be mediated by duct c ells with an apical Cl-/HCO3- exchanger acting in parallel with a cAMP-acti vated Cl- channel and protons being extruded through a basolateral Na+/H+ e xchanger. However, this may not be the only mechanism for HCO3- secretion b y the rat pancreas. Recently, several members of electrogenic Na+/HCO3- cot ransporters (NBC) have been cloned. Here we report the cloning of a NBC fro m rat pancreas (rpNBC). This rpNBC is 99% identical to the longer, more com mon form of NBC [pNBC; 1079 amino acids (aa); 122 kDa in human heart, pancr eas, prostate, and a minor clone in kidney]. The longer NBC isoforms are id entical to the rat and human kidney-specific forms (kNBC; 1035 aa; 116 kDa) at the similar to 980 C-terminal aa's and are unique (with different lengt hs) at the initial N-terminus. Using polyclonal antibodies to the common N- and C-termini of rat kidney NBC, a similar to 130-kDa protein band was lab eled by immunoblotting of rat pancreas homogenate and was enriched in the p lasma membrane fraction. Immunofluorescence and immunoperoxidase light micr oscopy of rat pancreatic tissue with both antibodies revealed basolateral l abeling of acinar cells. Labeling of both apical and basolateral membranes was found in centroacinar cells, intra- and extralobular duct, and main duc t cells. The specificity of the antibody labeling was confirmed by antibody preabsorption experiments with the fusion protein used for immunization. T he data suggest that rpNBC likely plays a more important role in the transp ort of HCO3- by rat pancreatic acinar and duct cells than previously believ ed. (C) 1999 Academic Press.